1. Field of the Invention
The present application relates to a method to tune an emission wavelength of a wavelength tunable laser diode (hereafter denoted as LD).
2. Related Background Art
As recent development in the wavelength division multiplexing (WDM) system, an LD installed therein as an optical signal source has been requested to tune an emission wavelength thereof precisely. A wavelength tunable LD is one of solutions for such an optical signal source. A wavelength tunable LD in an arrangement thereof provides an active region put between optical gratings each having a specific diffracting wavelength different from others, and the emission wavelength thereof may be adjusted to vary the diffracting wavelength.
Two techniques have been known to vary the diffracting wavelength of the grating, both of which vary the refractive index of the grating. The first technique changes a concentration of free carriers in the diffracting grating, while, the second technique changes a temperature there.